<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    div{

        
        width:  100px;
        height: 100px;
        border: 1px solid #333;
       /* 让一个块元素实现水平居中 */
            margin-left: auto;
            margin-right: auto;
        
    }
    
    </style>
</head>
<body>
    <!-- 标签名 + tab  -->
    <!-- 快速补全 -->

    <!-- ctrl + /  注释 -->
    <div></div>
</body>
</html>